1. Field of the Invention
The present invention generally relates to the processing of a tracking error signal in disk server system, and more particularly, to a method for generating a tracking error signal using a differential phase detection circuit.
2. Description of the Prior Art
Generally speaking, a pickup head is used in an ordinary optical pickup system to read data recorded on the disk tracks. Normally, there are many tracks on a disk and the pickup head moves between different tracks to read the data, the action of which is called cross-track motion. After the cross-track motion, pickup head needs and re-lock the track in order to read the data stably. When the pickup head cannot properly lock the track and read the data along the track, a tracking error is thus produced and the data cannot be read correctly.
FIG. 1 shows block diagram for generating DVD tracking error signal in a traditional disk system. Through the lens 104, the pickup head 102 focuses on a track on the surface of disk 100. And a quadrant photodetector 106 inducts signal A, signal B, signal C, and signal D. The oblique signals (signal A and signal C or signal B and signal D) go through I/V amplifier 108, 110 and are added to generate signal (A+C) and signal (B+D). Signal (A+C) goes through equalizer 112 and, after being equalized and amplified, goes through a comparator 116 for digitization. Similarly, signal (B+D) goes through equalizer 114 and comparator 118 to become digital signal.
There is almost no phase difference between the signal (A+C) and signal (B+D) when the pickup head 102 locks track correctly. Little variation among signal A, signal B, signal C and signal D inducted by the quadrant photodetector could be processed by a circuit to obtain a pickup head location. For instance: after signal (A+C) and signal (B+D) are received by phase detector 120, if the signal (A+C) is a phase lead signal, an UP clock is generated; otherwise, if signal (A+C) is phase delay signal, a DOWN clock is generated, as shown in FIG. 2. The up and down clock then go through a low pass filter 122 and 124 respectively and finally generate tracking error signal. In other words, when the crossing track of the pickup head 102 locks on the new track, no tracking error signal is generated because there is no phase difference between the signal (A+C) and signal (B+D). However, when the pickup head 102 has not completely locked, there is a phase difference between the signal (A+C) and signal (B+D), and thus a tracking error signal is generated. Optical systems adjust the pickup head location by the deviation of this tracking error signal to correctly lock the track.
According to Seong-Yun Jeong, Jung-Bae Kim and Jin-Yong Kim's report, “Analysis of DPD Signal Offset Caused by Optical Asymmetry” on SPIE Vol. 3109, the tracking error signal could be processed as the equation below, which means signals inducted by quadrant photodetector could be expressed by the equations below:A=a cos(ωt−ΦA)) B=b cos(ωt−ΦB) C=c cos(ωt−ΦC) D=d cos(ωt−ΦD)and the tracking error signal generated is       Tracking    ⁢                   ⁢    Error    =                    Phase        ⁡                  (                      A            +            C                    )                    -              Phase        ⁡                  (                      B            +            D                    )                      =                                        ΔΦ            CD                    +                                    c              a                        ⁢                          ΔΦ              AB                                                1          +                      c            a                              +                                    (                          1              -                              c                a                                      )                    ⁢                      (                                          Φ                A                            -                              Φ                C                                      )                                    (                      1            +                          c              a                                )                    
Wherein A, B, C and D are amplitude of signals inducted by 4 cells in a quadrant photodetector, a, b, c, and d, and ΦA, ΦB, ΦC, and ΦD, are, respectively, phase of their corresponding signals.
FIG. 3 shows that this sort of differential phase detection will be very sensitive to signal amplitude. FIG. 3 is to illustrate the influence on phase of the signal variation of a quadrant photodetector. For example, when the signal A on quadrant photodetector weakens, it causes bigger variation of the phase of signal (A+C). Thus, the variation of the signal becomes a miscarriage of judgment in relation to the phase variation and this affects the offset of tracking error signal and further affects the locking track point.
FIG. 1 shows the traditional phase detection circuit and the variation of the signal that causes misjudgment of phase and error for locking track point. FIG. 4 shows an improved phase detection circuit for generating a tracking error signal. The quadrant photodetector inducting signal A, signal B, signal C and signal D are digitized separately by different circuits. Separate cells induct phase difference that's caused by track crossing. This can avoid phase misjudgment from the signal variation and furthermore, accurately induct the lock track point.
However, in FIG. 4, the phase variation of signals inducted separately by quadrant photodetector is smaller and the signals make the phase signals generated by circuits that generate tracking error signal more sensitive to the phase delay and the phase lead which exist in the circuit, which can also create misjudgment of tracking error signal.